Magnetic Resonance Imagining (MRI) Magnetic Fields

1. Magnetic Resonance Imagining (MRI) Magnetic Fields • Protons in atomic nuclei spin on axes • Axes point in random directions across atoms • In externally applied magnetic field • Spin axes tend to align to magnetic field direction • Most “pointing” in same direction, but some “pointing” in opposite direction • How many align depends in part on strength of magnetic field

2. MRI Magnetic Fields Cont’d • In an externally applied magnetic field, atomic nuclei with an odd number of nucleons (protons + neutrons) precess • = Spin axis wobbles • Rate determined both by properties of the atom • And by the strength & homogeneity of the magnetic field • Hydrogen atoms in a 1.5 Tesla field precess at ~64 MHz = Radio frequency [rf] • Timing (phase) of wobble still random across atoms

3. Resonance • A radio frequency pulse (rf pulse) of a particular frequency is applied to the aligned protons in the magnetic field • When rf pulse frequency = precession frequency = Larmor frequency, nuclei resonate to it • Pushes spinning protons into phase with one another • Amplitude of wobble of the whole magnetic field generated by the spinning protons increases (= spin axis is pushed farther out) • Increases “transverse component” of magnetic field • Which is what’s measured • How far spin axis moves (= flip angle) depends on rf pulse intensity and duration

4. Why is this useful? • Takes characteristic amount of time after rf pulse for protons: • To get back out of phase (= de-phase) • And to settle back to original wobble amplitude in fixed field • Depending on what other kinds of atoms are nearby • i.e, the kinds of molecules the atoms are in, in part • As protons settle back into alignment with fixed field, the strength of the magnetic field they generated decreases • Variations in how long it takes the protons to de-phase & to settle back to original wobble amplitude in fixed field • Can be used to distinguish among different substances

5. How does this make IMAGING possible? • How know where magnetic field being measured comes from?) • Use gradient magnetic fields (Lauterbur Nobel Prize) • Generate field with gradation in field strength with only a small region at 1.5 Tesla • Only hydrogen atoms within that region respond to 64 MHz pulse • So, response must come from protons within 1.5 T region • Keep moving position of 1.5 T area to localize source of responses to repeated rf pulses • Size of 1.5 T region determines granularity of localization of response

6. Siemens Allegra 3TBiomedical Imaging Center (BIC)

7. Fixed field magnet is (almost) always on! • - Child killed in 2001 at Westchester • Medical Center when an oxygen tank • brought into magnet room was pulled • into center of magnetic field • In 2000, police officer’s gun pulled • from his hand into magnet & discharged • a bullet into the wall on the way in

8. Structural MRI • Anatomical scans generally measure hydrogen atoms in water • Since different kinds of tissue have different proportions of water • Typical anatomical scan voxel granularity = 1 x 1 x 3-5 mm

9. Functional MRI (fMRI) • Substance measured is hemoglobin (iron) in blood • Blood flow increases to active brain regions • Increases more than is usually needed • So ratio of de-oxygenated to oxygenated blood decreases • Oxygenated & de-oxygenated hemoglobin respond differently to magnetic field and rf pulses • Use this to detect where more oxygenated blood goes after some event • Takes several seconds for the response to peak • Timing seems to vary some across different brain regions • Fastest reliably detectable pre-peak response so far = 2 - 4 sec • Signal strength change very small – generally less than 1% change • If signal strength large, probably due to draining vein

10. Important to avoid measuring this instead of this

12. fMRI Cont’d • Spatial resolution: • Ultimate limit probably spatial specificity of the circulatory system • Worse than structural MRI • Typical functional scan voxels = 3 x 3 x 5 mm • Temporal resolution: • IF blood flow is what’s measured, never going to be faster than seconds • Working on detecting the brief initial decrease in oxygenated blood preceding increased blood flow • Working on imaging other substances

13. fMRI Data Analysis • Much of the brain is active much of the time • (e.g., “default network”) • Try to isolate regions that are specific to some aspect of the event of interest • One widely used approach is to look for voxels whose time course of signal strength change after stimulus is correlated with an idealized hemodynamic response function • Decide on a threshold for correlation strength and only further analyze voxels exceeding that threshold

14. Subractive Logic • Construct 2 conditions that you believe differ in just 1 important way • Treat one as baseline & subtract it from the other to get rid of all the • activity the 2 have in common • And then analyze what’s left • May only analyze part of what’s left because may threshold (again), • and/or may only analyze Regions of Interest (ROI) • These are all ways to cut down the number of statistical comparisons done

15. Subtractive Logic, Cont’d • Similar logic used in comparing conditions in most other kinds of experiments, too • But there’s been a very unfortunate tendency in much of the imaging literature so far, • For researchers who don’t have a good understanding of the many ways that different kinds of stimuli and/or tasks and/or situations can differ, • To claim that they’ve located “phonological word processing”, or “irregular morphological inflection processing”, or some such aspect of language processing • When other confounded differences between conditions are equally good candidates (such as plain old difficulty) for explaining the effects